The present invention relates generally to the field of double fed induction generators. In particular, the present invention relates to control and protection of double fed induction generators during grid faults.
Double fed induction generators (DFIGs) are used in a wide range of applications because of their efficiency and reliability. Generally, DFIGs are used in variable speed generation (VSG) systems for generating electric energy from intermittent or variable energy resources such as wind farms. The main advantage of a variable speed generation system over a fixed speed system is the possibility of electronically controlling the shaft speed in order to maintain maximum efficiency of the energy conversion process. For example, a wind turbine generator typically uses a DFIG comprising an AC/DC converter coupled to a DC/AC converter for wind power generation. The DFIG technology enables maximum energy to be extracted from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind and grid transients. Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus reducing the need for installing capacitor banks.
DFIG controller design considerations have generally concentrated on providing an adjustable operating speed to maximize turbine power output, maintaining the required generator terminal voltage or power factor, and controlling the generator torque to match that of the wind turbine. However, little or no attempt has been made to provide the capability of contributing to stability of power network operation. The increasingly widespread use of wind power generation requires the wind-farms to contribute to the stability of power network operation.
Wind turbines with double fed induction generators are sensitive to grid faults. A grid fault will give rise to severe transients in the air-gap torque and shaft torque and may therefore impact system reliability. For example, when grid voltage drops below a threshold value due to a grid fault, the air-gap torque also drops, thereby leading to an oscillation in the gearbox and drive shaft that may reduce gearbox life. Grid recovery sequences also result in severe transient conditions in double fed induction generators. A conventional technique to avoid damage in the DFIG and the gearbox includes disconnecting the wind turbine generators from the grid when large voltage sags appear in the grid due to grid fault. After a period of time, the turbine is then reconnected to the grid. However, new grid codes now require wind turbines and wind farms to ride through voltage sags, meaning that normal power production should be immediately re-established once the nominal grid voltage has recovered.
Different techniques have been proposed to modify the DFIG system so as to achieve above requirement. For example, anti-parallel thyristors may be used in the stator circuit to achieve a quick (within 10 milliseconds, for example) disconnection of the stator circuit, and also provide the capability to remagnetize the generator and reconnect the stator to the grid as fast as possible. Another option proposed is to use a static switch in the rotor circuit, which can break the short circuit current in the rotor. A third method is to use a DVR (Dynamic Voltage Restorer) that can isolate the DFIG system from voltage sags. However, the first two options are not effective in reducing shaft stress, and the third option is expensive.
It is therefore desirable to provide a control and protection technique that enables a DFIG to efficiently contribute to power network operation and reduce the shaft stress during grid faults.